Refractometer

ABSTRACT

This invention relates to an improved recording refractometer of the critical angle type wherein the brightness of the dark field is observed continuously by one reference photocell while another photocell assembly (measuring) scans to find the light-dark dividing line. The electrical outputs of the reference and measuring photocells are coupled to a bridge circuit, amplifier and servo motor which pivots the measuring photocell assembly until the slit adjacent to the measuring cell comes into, and remains in, coincidence with the light-dark boundary. Indicating or recording means coupled mechanically to the measuring photocell assembly shows the refractive index of the sample.

[75] Inventor:

United States Patent 1 Barstow [52] US. Cl. 356/131 [51] Int. Cl. G0ln21/46 [58] Field of Search 356/131, 132, 135,

[56] References Cited UNITED STATES PATENTS 2,624,014 12/1952 Barstow356/135 9/1971 l-lalm 250/210 11 3,738,757 June 12, 1973 PrimaryExaminer-Ronald L. Wibert Assistant Examiner-Conrad Clark A ttorney-Earl D. Ayers and Griswold & Burdick [57] ABSTRACT This inventionrelates to an improved recording refractometer of the critical angletype wherein the brightness of the dark field is observed continuouslyby one reference photocell while another photocell assembly (measuring)scans to find the light-dark dividing line. The electrical outputs ofthe reference and measuring photocells are coupled to a bridge circuit,amplifier and servo motor which pivots the measuring photocell assemblyuntil the slit adjacent to the measuring cell comes into, and remainsin, coincidence with the lightdark boundary. Indicating or recordingmeans coupled mechanically to the measuring photocell assembly shows therefractive index of the sample.

10 Claims, 3 Drawing Figures 7'07 a/ f/ec f ing l fldm 5052 fye a W l .5i 360/6 Eye lens /en$ l l Reference 8 i5 I c)e///mea;

60m .S/l'f i 9 Wanua/[ya0juJ/e40 Ph0 7 0 ce// reference 1 REFRACTOMETERBACKGROUND OF THE INVENTION This invention relates to refractometers andparticularly to recording refractometers of the critical angle type.

Other recording refractometers of the type are known in the art. One,for example, is described in U.S. Pat. No. 2,624,014 to O. Barstow.

Such devices commonly have standardizing mechanisms including timers,solenoids, clutches, potentiometers, for example. Also, measuring isoften interrupted when standardizing.

In at least some of the prior art devices, there is a time delay duringuse of the device to allow the photocell to come to equilibrium aftergoing from the dark to light field and vice versa.

OBJECTS OF THE INVENTION Accordingly, a principal object of theinvention is to provide an improved refractometer of the criticalanglety e.

Another object of this invention is to provide an improved, simpler toconstruct and operate refractometer of the critical angle type.

A further object of this invention is to provide an improvedrefractometer of the critical angle type which requires no interruptionof the measuring function during dark field measurement.

Still another object of this invention is to provide an improvedrefractometer of the critical angle type which provides compensation forlight intensity changes.

. STATEMENT OF THE INVENTION In accordance with this invention, there isprovided refractometer apparatus including a suitable light source fromwhich light is directed at a sample receiving vessel in which the samplecontacts a surface of a refracting prism, passes through the prism and alens, and impinges on a partially reflecting plate which reflects aportion of the light to a reference photocell assembly. The remainder ofthe light from the prism and lens passes through the reflecting plateand impinges on a second reflecting element and thence through a slitand onto a measuring photocell. The second reflecting element, slit andmeasuring photocell are part of a moveable assembly.

The electrical outputs of the photocells are coupled to a bridgecircuit, amplifier and servo motor which pivots the moveable assemblyuntil the slit adjacent to the measuring cell comes into, and remainsin, coincidence with the light-dark boundary. Indicating or recordingmeans coupled to the moveable assembly critical angle a showsthe'refractive index of the sample.

BRIEF DESCRIPTION OF THE DRAWING The invention, as well as additionalobjects and advantages thereof, will best be understood when thefollowing detailed description is read in connection with theaccompanying drawing, in which:

FIG. 1 is a diagrammatic view of mechanical and optical apparatus inaccordance with this invention;

FIG. 2 is a basic, simplified schematic diagram of bridge and slit andpen positioning apparatus for use with the apparatus of FIG. 1, and

FIG. 3 is a more complex circuit suitable for use with the apparatus ofFIG. 1.

Referring to the drawing, and particularly to FIG. 1, there is shown alight source 10, such as an incandescent bulb, for example, enclosed ina suitable housing 12 having a window 14 facing the first surface 16 ofa refracting prism 18 which is enclosed in a liquid-tightsample-receiving housing 20.

A band pass filter 22, objective lens 24 and a clear glass plate orsemi-silvered beam-splitting mirror element 26 are spaced from andaligned along the longitudinal axis of the refracting prism 18. Thefilter 22 and objective lens 24 are disposed generally perpendicularlywith respect to the longitudinal axis of the prism 18 while the plate 26is tilted, at 45, for example, with respect to the longitudinal axis ofthe prism 18.

A total reflecting prism 28 is disposed beyond and axially aligned withthe filter 22 and objective lens at a distance more remote from therefracting prism 18 than is the plate 26.

A slit assembly 30 and measuring photocell 32 are disposed with respectto the prism 28 so that light from the prism 18 which passes through thefilter 22, lens 24 and plate 26 is reflected by the prism 28 and focusedin the plane of slit 30, some of the light passing through the slit tothe photocell 32.

The prism 28, slit 30, and the measuring photocell 32 are mounted on arigid assembly 34 which is pivoted from a point 36 near the objectivelens.

A slit 38 and reference photocell 40 is aligned to receive lightreflected from the plate 26.

A scale lens 50 and eye lens 52 are aligned along the axis of the prism18 and objective lens 24 so that, for checking purposes, an individualmay use his eye 54 to visually observe the position of the light-darkboundary, characteristic of a critical angle refractometer, relative toa vertical scale engraved on the scale lens, after the assembly 34 ispivoted out of line.

Referring now to FIG. 2, there is shown a bridge circuit, indicatedgenerally by the numeral 56, composed of photosensitive elements 32, 40as two legs of the bridge and resistor 58 and resistors 60a, 60b as theother two legs of the bridge. A voltage source, such as the battery 62,is coupled across the junction between the photoresistive elements 32,40 between meters 64 66 and between the resistors 58 and 60a.

With switch 88 in the position shown, these two latter legs are equal inresistance since resistor 58 equals the sum of resistors 60a and 60b.These bridge legs have a ratio of 1:1 as indicated in the diagram andthe signal to the amplifier will be zero if the currents through thephotocells are equal. With switch 88 in the position marked 2:1, thebridge arms have a ratio of 2:1 since the resistance values bear therelation:

Resist 58 Resist 60a 2 Resist 60b.

Now the current through photocell 32 has to be twice that throughphotocell 40 to bring the amplifier signal to zero. The motor will drivethe slit 30 (by driving pivoted assembly 34) until it reaches thelight-dark boundary and this 2:1 ratio in current exists.

An amplifier 68 has its input coupled across the junctions between thephotoresistive elements and the adjacent resistors (58 adjacent toelement 40 and 60b adjacent to element 32.) The output of the amplifier68 is coupled to servo motor 70 which controls the movement of theassembly 34 in FIG. 1 (shown symbolically in FIG. 2 by the dashed line72 to measuring slit 30) and of the recorder pen 74.

In FIG. 3, the bridge circuit is essentially that of FIG. 2 except thata tapped resistor 60c replaces resistors 60a, 60b and power from thebattery 62a is applied to the junction between the photoresistiveelements 32, 40 through two poles of a double pole, triple throw switch,indicated generally by the numeral 76.

When the switch 76 is coupled to the pair 78a, 78b, the resistance 80serves to limit current from the battery 62a, the meter 82 thus beingused as the indicator means of a battery check device. In fact, with asuitable meter and resistance 80, the meter can read the voltage outputof battery 62a.

When the switch 76 is coupled to pair 84a, 84b, voltage is appliedacross the photoresistor elements 32 and 40 with the microammeter 82measuring the current through the photoresistor element 40.

When the switch is connected to pairs 86a, 86b, voltage is applied tothe photoresistor elements 32 and 40 with the meter 82 measuring thecurrent through photoresistor element 32.

In operation, liquid whose refractive index is to be measured is broughtinto the housing 20. Usually the housing is provided with athermostatically controlled bath to hold the sample temperature at apredetermined temperature.

Light from the source passes through the sample, some of it striking theface 16 of the prism 18 at grazing incidence, and some of it strikingthe face 16 at a slight angle.

With the face of the prism covered with sample liquid and thusilluminated, an observer looking from the position of the eye 54, forexample (the assembly 34 being depressed manually so as to provide anunobstructed path for the light emerging from the objective lens to theeye piece), sees a field in which the upper portion is light and thelower portion is dark. The horizontal dividing line between the light(upper portion) and dark (power portion) of the split field is referredto usually by the term light-dark boundary and is delineated by theobjective lens bringing the rays of the refracted light beam to a focusin the plane of the scale lens.

The vertical position of the light-dark boundary on the scale lensdepends upon the index of refraction of the material of the prism 18,the angle of the prism face 16 with respect to the prism axis and theindex of refraction of the sample. For a given refractive index of thesample, the prism 18 may be chosen (angle and refractive index) to bringthe light-dark boundary to the center of the scale lens or roughly toany position on the scale lens desired.

The eye lens 52 serves as a magnifier to permit accurate reading of theposition of the light-dark boundary with respect to the scale engravedon the scale lens. The reading thus obtained can, by suitablecalibration, be interpreted to give sample refractive index.

The brightness of the light portion of the split field compared to thatof the dark portion is a variable but oftentimes is in the order of 10to l and may go as high as 100 to 1.

Part of the light (about 8 percent, for example) passing through theprism 18, filter 22 and objective lens is reflected downwardly by theplate 26, passing through the relatively wide reference slit 38 andimpinging on the photocell 40.

The remainder of the light impinging on the plate 26 passes through andis reflected by reflecting element 28 downwardly and focuses in theplane of slit 30 which 4 is in fixed position with respect to the prism28. The light focused in the plane of slit 30 forms a light-dark fieldin this plane with the light portion on'the left in FIG. 1 and the darkportion on the right.

Likewise, the light reflected by plate 26 is focused in the plane of thereference slit 38 forming in this plane, the light-dark field with thelight portion on the left and the dark portion on the right in FIG. 1.

Note that using total reflecting prism 28 and positioning slit 30 andphotocell 32 as shown in FIG. 1 is equivalent optically to putting slit30 in the plane of the scale lens and photocell 32 to the right of theslit without the total reflecting prism; and that rocking of the pivotedassembly about pivot 36 is then equivalent to moving the slit up anddown across the light-dark field in the plane of the scale lens with thephotocell moving up and down with the slit (or more accurately, movingin a slight arc about point 36).

Refer to FIGS. 1 and 2 in which the switch 88 is connected to thejunction between resistors 58 and 60a (the 1:1 position) and assume thatthe reference slit 38 is adjusted manually so that it is in the darkfield.

To balance the bridge, the measuring slit is first moved (by rockingassembly 34) into the same part of the dark field occupied by thereference slit and the opening in the reference slit is adjusted so thatthe readings of meters 64, 66 are equal, or until the motor stops. Thereference slit must be opened wider than the measuring slit because amuch smaller amount of light is deflected by plate 26 to the referenceslit than is deflected to the measuring slit 30 by the prism 28.Normally this balancing of the bridge does not need to be repeatedunless the dark field brightness changes markedly with time.

Once the bridge is balanced, the switch 88 (or 88a in FIG. 3) is set tothe point marked 2:1. The bridge is now not balanced and the motor 70,driven by the amplifier 68, will run, driving slit 30 toward the lightfield until the input to the amplifier is zero which will occur when theslit starts to cross the light-dark boundary. Then it will stop and thepen will record the refractive index of the sample on a suitable chart.If the refractive index changes the slit will be driven one way or theother to keep it in coincidence with the light-dark boundary, and thepen will continue to read the refractive index of the sample.

The ratio of 2:1 4:1 or more of the resistance legs of the bridge ischosen to give optimum performance of the servo system in positioningthe slit on the lightdark dividing line and depends on relativebrightness of the light and dark fields and sharpness of the dividingline.

The circuit of FIG. 3 permits checking the current from both thephotoresistive elements and the voltage from the power supply with onlyone meter. Also, a greater number of taps on the resistance legs of thebridge permit operation over a wider range of light intensities in thesplit field.

Other readout devices than a pen 74 may be used in connection with theapparatus, as will be recognized by those skilled in the art.

What is claimed is:

1. Refractometer apparatus comprising a stationary light refractingprism for refracting a beam of light, said prism having a lightreceiving surface and a light emergent surface, said light receivingsurface being adapted to being covered by a sample whose refractiveindex is to be obtained, a light source opposite the light receivingsurface adapted to direct a beam of light through the sample onto thelight receiving surface at grazing incidence to produce a refractedlight beam, said refracted light beam having a dark area, a light areaand a light-dark boundary on being brought to a focus, means adapted tobring to a focus the refracted light so as to delineate the said darkarea, light area and lightdark boundary, reference light sensitive andlight limiting means, said light sensitive means being aligned with saidlight limiting means and being adapted to develop an electrical outputsignal in response to impingement thereon of light from said refractedlight beam, fixed positioned light deflecting means disposed in the pathof said refracted light for directing focused refracted light from saiddark area to said reference light limiting means and light sensitivemeans, measuring light limiting and light sensitive means, saidmeasuring light sensitive means being aligned with said measuring lightlimiting means and being adapted to develop an electrical output signalin response to impingement thereon of light from said refracted lightbeam, means for scanning light from said refracted light beam acrosssaid measuring light sensitive means, circuit means for comparing theoutput signals from said two light sensitive means and for deriving anoutput which is a function of the two signals, amplifier meansresponsive to said circuit output and coupled to said circuit, a servomotor, said servo motor being coupled to said amplifier means, saidmotor being controlled by said servo amplifier and being mechanicallycoupled to said means for scanning and to readout means, said servoamplifier controlling the movement of said means for scanning until saidcircuit output is zero as a result of the light impinging on saidreference and measuring light-sensitive means, said circuit output beingzero when said measuring light limiting means coincides with saidlight-dark boundary.

2. Refractometer apparatus in accordance with claim 1, wherein saidmeans adapted to bring to a focus the refracted light is a lens.

3. Refractometer apparatus in accordance with claim 1, wherein saidreference light limiting means is a manually adjustable slit element.

4. Refractometer apparatus in accordance with claim I, wherein saidreference and measuring light sensitive means are photo-resistiveelements.

5. Refractometer apparatus in accordance with claim 1, whereinsaid-readout means is a recorder whose pen is mechanically coupled tosaid motor.

6. Refractometer apparatus in accordance with claim 1, wherein saidcircuit means is a bridge circuit including a pair of resistance armsand said reference and light sensitive means and measuring lightsensitive means forming a pair of opposing arms thereof and said servomotor has its input coupled across each junction between a lightsensitive means and a resistive arm.

7. Refractometer apparatus in accordance with claim 1, wherein anadditional light deflecting means is disposed in the path of saidrefracted light beam to thereby deflect light to said measuring lightlimiting and sensitive means.

8. Refractometer apparatus in accordance with claim 7, wherein saidmeans for scanning includes pivotal mounting means for rotating saidadditional light deflecting means and said measuring light limitingmeans and light sensitive means around a common axis, near to said meansadapted to bring said refracted light to a focus.

9. Refractometer apparatus in accordance with claim I, wherein both saidlight limiting means are slit elements.

10. Refractometer apparatus in accordance with claim 1, wherein thecircuit for comparing the signals from the two light sensitive means isadjustable so the relation between the signals from the two lightsensitive means at the balance point may be varied.

*zgz gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pate3,738,757 I Dated June 12, 1973 Inventor(s) Ormond E. Barstow It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, line 40, change "photosensitive" to photoresistive Signed andsealed this lhth day of May. 197).

(SEAL) Attest:

R ETCHER JR. I c. MARSIiALL DA'NN fig sii ri orricer Commissioner ofPatent

1. Refractometer apparatus comprising a stationary light refractingprism for refracting a beam of light, said prism having a lightreceiving surface and a light emergent surface, said light receivingsurface being adapted to being covered by a sample whose refractiveindex is to be obtained, a light source opposite the light receivingsurface adapted to direct a beam of light through the sample onto thelight receiving surface at grazing incidence to produce a refractedlight beam, said refracted light beam having a dark area, a light areaand a light-dark boundary on being brought to a focus, means adapted tobring to a focus the refracted light so as to delineate the said darkarea, light area and light-dark boundary, reference light sensitive andlight limiting means, said light sensitive means being aligned with saidlight limiting means and being adapted to develop an electrical outputsignal in response to impingement thereon of light from said refractedlight beam, fixed positioned light deflecting means disposed in the pathof said refracted light for directing focused refracted light from saiddark area to said reference light limiting means and light sensitivemeans, measuring light limiting and light sensitive means, saidmeasuring light sensitive means being aligned with said measuring lightlimiting means and being adapted to develop an electrical output signalin response to impingement thereon of light from said refracted lightbeam, means for scanning light from said refracted light beam acrosssaid measuring light sensitive means, circuit means for comparing theoutput signals from said two light sensitive means and foR deriving anoutput which is a function of the two signals, amplifier meansresponsive to said circuit output and coupled to said circuit, a servomotor, said servo motor being coupled to said amplifier means, saidmotor being controlled by said servo amplifier and being mechanicallycoupled to said means for scanning and to readout means, said servoamplifier controlling the movement of said means for scanning until saidcircuit output is zero as a result of the light impinging on saidreference and measuring light-sensitive means, said circuit output beingzero when said measuring light limiting means coincides with saidlight-dark boundary.
 2. Refractometer apparatus in accordance with claim1, wherein said means adapted to bring to a focus the refracted light isa lens.
 3. Refractometer apparatus in accordance with claim 1, whereinsaid reference light limiting means is a manually adjustable slitelement.
 4. Refractometer apparatus in accordance with claim 1, whereinsaid reference and measuring light sensitive means are photo-resistiveelements.
 5. Refractometer apparatus in accordance with claim 1, whereinsaid readout means is a recorder whose pen is mechanically coupled tosaid motor.
 6. Refractometer apparatus in accordance with claim 1,wherein said circuit means is a bridge circuit including a pair ofresistance arms and said reference and light sensitive means andmeasuring light sensitive means forming a pair of opposing arms thereofand said servo motor has its input coupled across each junction betweena light sensitive means and a resistive arm.
 7. Refractometer apparatusin accordance with claim 1, wherein an additional light deflecting meansis disposed in the path of said refracted light beam to thereby deflectlight to said measuring light limiting and sensitive means. 8.Refractometer apparatus in accordance with claim 7, wherein said meansfor scanning includes pivotal mounting means for rotating saidadditional light deflecting means and said measuring light limitingmeans and light sensitive means around a common axis, near to said meansadapted to bring said refracted light to a focus.
 9. Refractometerapparatus in accordance with claim 1, wherein both said light limitingmeans are slit elements.
 10. Refractometer apparatus in accordance withclaim 1, wherein the circuit for comparing the signals from the twolight sensitive means is adjustable so the relation between the signalsfrom the two light sensitive means at the balance point may be varied.